If nLocktime is nonzero and below 500 million, it is interpreted as a
block height, meaning the transaction is not valid and is not relayed
or included in the blockchain prior to the specified block height. If
it is above 500 million, it is interpreted as a Unix Epoch timestamp
(seconds since Jan-1-1970) and the transaction is not valid prior to
the specified time. Transactions with nLocktime specifying a future
block or time must be held by the originating system and transmitted
to the bitcoin network only after they become valid. If a transaction
is transmitted to the network before the specified nLocktime, the
transaction will be rejected by the first node as invalid and will not
be relayed to other nodes.